Abstract: A soil engineering classification derived from the piezocone penetration test(CPTU) involves the uncertainty of correlation between soil composition and soil mechanical behavior.This uncertainty results in overlaps of different soil types in currently used CPTU soil classification systems.The cluster analysis is a statistical method for grouping similar mathematical data sets and is used herein for illustrating the soil stratigraphy from CPTU data.Its application to defining stratigraphic interfaces is illustrated using five expressway case studies with layered vertical profiles.It is concluded that the cluster analysis is not only able to delineate major changes within the stratigraphy but also detect thin layers within a sublayer.Therefore,the cluster analysis can provide valid and reliable preliminary soil stratification based upon the CPTU original data.